1. Field of the Invention
The invention relates to circular knitting and more particularly to closed track camming for such machines.
2. Description of the Prior Art
The increase of knitting rate of circular knitting machines is limited by the extent of acceleration and of acceleration changes to which the cams and needles are subjected as well as by the tractions applied to the yarn. The needle of a circular knitting machine is displaced at constant angular speed around the longitudinal axle of the needle bed and at variable speeds in a direction parallel to this axle. This variable speed is given to the needles by means of cams fastened around the needle bed. In the past, such cams were in the form of successively ascending and decending linear track sections linked by connecting portions wherein the needles had a degree of freedom. Each time a needle was actuated by such cam, it was subjected to a very high acceleration and to a nearly infinite acceleration change (pulse). These high values lead to rapid wear of the needle butts and limited the spinning of the needle bed to relatively low r.p.m.
One has already proposed to remedy these drawbacks by using cams forming a closed cam track for ensuring constant control of the needle displacements and by linking the ascending and descending track portions by arcuate portions. Such design reduces the accelerations to lower values and decreases the stresses experienced by the needles and the cams.
As a result, considerable improvements, for instance, lower wear of needles and cams as well as increased rotations rates were achieved.
As it is known from general theories in the art of cam making, curves corresponding to equations comprising 6th degree polynomials are capable of producing steady accelerations and acceleration changes. However, in the case of cams for knitting machines, although such types of solutions may first appear the most satisfactory regarding needle life, they are plagued by other drawbacks inherent to stitch formation and overtensions in the yarn. Further, such cam shapes will limit unacceptable the number of feeds/turn because of their size or they will require that very steep slopes be used (about 60.degree. or more). It should also be recalled that although the influence of cam shape on yarn tensions and wear of needles and cams has been much investigated, the fact that cam track, which is formed by a plurality of cam track sections of track blocks separated from each other, is discontinuous should not be overlooked. Even if each cam track section is designed to steadily accelerate the needles, the passing of said needles from one cam track element to the following one will produce a sudden and possible significant change of speed. This results from the fact that, when passing from two adjoining elements, the needles are out of control for a short period. Their vertical movement during such period which is determined by the rubbing forces against the walls and the tension in the yarn cannot be controlled accurately with the result that, when reaching the next cam track portion, the needle undergoes an acceleration of undetermined magnitude. Consequently, even when using cam track sections for imparting steady acceleration changes to the needles, the latter may be subjected to large speed variation when passing from one track section to the next one. The present discussion is intended to show that it is not necessarily desirable that each cam section be calculated as a function of a steady acceleration variation if the needle is still subjected to knocks when jumping from one to the other track section.